Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface showing my new design, the graphical user interface shown separately for ease of illustration; and,

FIG. 2 is a front view thereof, shown with environment.

The broken line showings of the text, numbers, symbols, and images in FIGS. 1 and 2 and the mobile device in FIG. 2 are included for the purpose of illustrating portions of the article and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 